German Pat. No. 569 473 acknowledges a method for cleaning silver with the aid of an aluminum contact agent in alkaline solutions, for example solutions containing sodium bicarbonate with or without the addition of soap solutions, and discloses the addition of aldehyde sugar to a solution containing sodium bicarbonate and soap, more particularly 10 g of a mixture comprising 92.5% NaHCO.sub.3, 5% powdered medicinal soap and 21/2% grape sugar dissolved in 1 liter of water in an aluminium vessel. However, this not only involves having to measure out the four constituents with considerable accuracy, it also results in damage to the aluminium container, even to such an extent around its bottom as to cause it to be holed and become useless.
Tarnish, silver sulphide (Ag.sub.2 S), is formed as a very thin layer on the surface of silver or silver-plated articles due to the action of hydrogen sulphide (H.sub.2 S), which is present in the air and also in some mineral waters, according to the equation: EQU 2Ag+H.sub.2 S=Ag.sub.2 S+H.sub.2
Silver sulphide is the least soluble in water of all the silver compounds. However, it can be readily reduced by contact with aluminium in dilute sodium carbonate (Na.sub.2 CO.sub.3) solution, the reduction resulting from the liberation of hydrogen. As sodium carbonate is the salt of a strong base and a weak acid it dissociates to give an alkaline solution.
Hydrogen is likely to be produced via sodium hydroxide (NaOH) as an intermediate according to the equation: EQU Na.sub.2 CO.sub.3 +2H.sub.2 O=H.sub.2 CO.sub.3 +2NaOH
The sodium hydroxide then reacts with the aluminium to give soldium aluminate (NaAlO.sub.2) and hydrogen: EQU 2NaOH+Al=NaAlO.sub.2 +H.sub.2
The sodium aluminate appears to ionise as a 1:1 electrolyte: EQU NaAl(OH).sub.4 (H.sub.2 O).sub.2 =Na.sup.+ +[Al(OH).sub.4 (H.sub.2 O).sub.2 ]
The hydrogen evolved will be `nascent` and, therefore, particularly active as a reducing agent, reduction occurring (obviously) according to the equation: EQU Ag.sub.2 S+H.sub.2 =2Ag+H.sub.2 S
the hydrogen sulphide released acting as a weak dibasic acid and being absorbed by the alkaline medium to form either of two salts--sodium sulphide (Na.sub.2 S) and sodium hydrogen sulphide (NaHS)--depending upon the amount of hydrogen sulphide present: EQU 2NaOH+H.sub.2 S=Na.sub.2 S+2H.sub.2 O EQU NaOH+H.sub.2 S=NaHS+H.sub.2 O